I do believe that in a lot of cases people would be better served by measuring it first. I live in central London near a small street on the first floor and mea

I do believe that in a lot of cases people would be better served by measuring it first. I live in central London near a small street on the first floor and measured PM2.5 for a year, it barely ever went over 2ug/m3, with 10ug/m3 being safe long term threshold set by the WHO, which makes any PM2.5 air purification useless. Now, NO2 is a bit different and I haven't measured it, but common household air purifiers do nothing for it.